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Tree's weapon against elephants? Ants!

September 4, 2010 | 9:57
am

Ecologists have discovered the secret weapon used by certain acacia trees to defend themselves against ravenous elephants: ants.

The finding could one day help conservationists protect vulnerable plants from elephants and other large herbivores, said University of Florida biologist Todd Palmer, who reported the discovery online Thursday in the journal Current Biology.

Elephants can have a devastating impact on the trees of the African savannas, Palmer said. A hungry pachyderm can easily demolish a tree, wrapping its prehensile trunk around thick branches and ripping them off. A herd of them can lay waste to an area -- a problem for people trying to protect wild lands or cropland.

Yet elephants always seem to avoid one particular type of acacia tree called Acacia drepanolobium, also known as the whistling-thorn tree.

What sets these acacias apart is the ants that inhabit them. The insects live in the trees' golf-ball-sized "swelling thorns" and feed on the nectar that oozes from the base of each leaf, Palmer said. In return, they provide the tree with bodyguard service, swarming out of the tree to face any attack.

Palmer guessed the elephants were turned off by the aggressive ants. He had once witnessed a rare instance of elephants attempting to eat the whistling-thorn trees during a very dry year, when food was scarce.

"The elephants were pulling up very small trees -- under a meter tall -- picking them up, putting them down on the ground and scraping the trees under their feet," he recalled. They were trying to remove the ants.

The elephants' weakness was the insides of their trunks, where the skin is filled with sensitive nerve endings — and where biting, stinging ants can inflict the most pain.

To test whether the ants were truly deterring the pachyderms, Palmer and University of Wyoming ecologist Jacob Goheen fed some young orphaned elephants branches from whistling-thorn trees, as well as from another acacia. When there were no ants on the branches, the elephants were just as likely to eat Acacia drepanolobium as they were their usual tree food. But when the branches held ants, the elephants avoided both types of acacia entirely.

Palmer said the ants probably warned the elephants of their presence by emitting an acrid stench.

If it were possible to identify and replicate the chemical compound that produced the stench, he added, it could potentially be used to protect crops and other at-risk plant life.

David Augustine, an Agricultural Research Service ecologist in the U.S. Department of Agriculture, said the next step would be to determine why other species of acacia trees don't host ants that are such effective bodyguards.